This invention relates to circuit components mounted on circuit boards by solder column interconnects or arrays, and more particularly, to support of circuit components having column grid arrays.
Solder column interconnects or arrays are used to interconnect circuit packages to circuit boards, most typically large ceramic integrated circuit packages. One such package is the column grid array (CGA) integrated circuit package that has a ceramic substrate with an array of solder columns that extend out from it, typically from the bottom surface. The solder columns are attached at one end to connection pads or locations on the ceramic substrate. The solder columns are sufficiently tall so that when the circuit package is placed on a circuit board, the solder columns can accommodate the difference in thermal expansion between the ceramic integrated circuit package and the printed circuit board. The solder columns are soldered to respective pads on the circuit board using known techniques, such as a convection reflow solder process.
One problem with solder column arrays is that the solder columns do not withstand compressive force well. The solder columns in the solder column arrays are typically made of a 90%/10% Pb/Sn solder, making them soft. Moreover, the individual solder columns are very thin. Consequently, if any significant amount of compressive force is applied to the circuit package, such as might be applied by a cooling solution such as a heat sink or fan, the resulting weight on the circuit package may cause the solder column array to compress, particularly over time, which may compromise reliability. In this regard, loads in excess of about 10 to 20 grams per column exert sufficient compressive force so that reliability may be compromised. Since CGA integrated circuit packages are being increasingly used for high power integrated circuits, the cooling solution required often has considerable mass. It is not unusual for the heat sink or cooling solution used with such high power integrated circuits to exceed one pound.
In accordance with an aspect of the invention, a circuit board assembly has a circuit board and an integrated circuit package. The integrated circuit package has a substrate having an array of solder columns extending from a bottom surface. The integrated circuit package has a lid that is affixed to top of the substrate. The lid is oversized with respect to the substrate so that it has a portion that extends beyond the outer periphery of the substrate. In an aspect of the invention, the periphery of the lid is larger than the outer periphery of the substrate so that the portion of the lid extending beyond the outer periphery of the substrate extends around the outer periphery of the substrate. Supports shims re disposed between the portion of the lid extending beyond the outer periphery of the substrate and portions of a circuit board when the CGA integrated circuit package is mounted on the circuit board. The support shims support the column grid array integrated circuit package against compressive force, such as would be exerted by a cooling solution mounted on the CGA integrated circuit package, and are made of sufficiently rigid material to do so, such as metal or plastic.
In an aspect of the invention, the support shims are disposed between the portion of the lid extending beyond the periphery of the substrate and the portions of the circuit board after the integrated circuit package is mounted on the circuit board and affixed to the lid by adhesive. The adhesive accommodates any variation in height of the integrated circuit package.
In an aspect of the invention, the integrated circuit package is rectangular and the shims are corner shims having first and second leg sections at generally right angles to each other with a support shim disposed at each corner of the integrated circuit package.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.